This invention relates to a method for differentiating tissues in Magnetic Resonance Imaging (MRI), and more particularly, to a method for quantitatively differentiating tissues that are rich in macromolecules from those containing small molecules (e.g. with a molecular weight of about 3K Daltons), in Magnetic Resonance Imaging.
Tissue differentiation and localization always have been basic goals of magnetic resonance imaging. Over the years, attempts to distinguish between tissues that are rich in macromolecules and tissues that contain small molecules have resulted in several quantitative techniques. Known methods of such quantitative techniques include T2-weighted imaging, Magnetization Transfer (MT) weighted imaging, Diffusion Weighted Imaging (DWI) etc., which involve creating different signals for tissue differentiation. However, these signals also contain either T1 or T2 weighting, and therefore, interpretation of these images requires more holistic thinking, understanding the effect(s) of pathology on these additional weighting factors and their effects on the signal.
Imaging contrast generated with magnetization transfer (MT) technique is dependent on the phenomenon of magnetization exchange between semisolid macromolecular protons and water protons. This technique has the ability to indirectly image the presence of semisolids such as protein matrices and cell membranes whose magnetization dies away too quickly to be imaged directly. MT contrast (MTC) has proved to be a useful diagnostic tool in characterization of a variety of central nervous system pathologies including infection, demyelination, and other neurodegenerative conditions. Although Magnetization Transfer (MT) imaging eliminates the T2 effect, T1 effects are apparent and found to affect SNR (signal-to-noise ratio) of the image.
Transverse signal relaxivity (R2) maps are not very sensitive and typically require a large effect to occur before indicating the change.
Diffusion Weighted-Echo Planar Imaging (DW-EPI) is known for imaging evaluation of intra-cranial tumors and also to detect and distinguish between acute hemorrhagic and non-hemorrhagic strokes.
Diffusion measurements in vivo are useful for tissue characterization as they provide information on the mobility of water or cell metabolites. MR images can be sensitized to diffusion by means of large magnetic field gradient pulses, allowing non-invasive estimation of apparent diffusion coefficient (ADC). Disruption of the permeability or geometry of structural barriers by pathology alters the diffusion behavior of water molecules. Thus, characterizing diffusion in the brain provides a technique to investigate the effects of disease processes on tissue microstructure.
At present, the most important clinical application of diffusion weighted imaging (DWI) technique is in the detection and characterization of cerebral ischemia. DWI is gradually being used in the evaluation of other intra-cranial pathologies like tumors, abscess and encephalitis. DWI is exquisitely sensitive to axonal directionality in the brain white matter and used for the study of demyelination due to trauma or axonal disruption, which is taken advantage of in the increasing applications of diffusion tensor imaging (DTI). DW-EPI has also enabled in vivo measurement of diffusion in abdominal organs such as liver and kidneys.
Apparent Diffusion Coefficient (ADC) maps and choline maps, although independent of T1 and T2 effects and indicate macromolecular content in quantitative fashion, are found to vary making the changes found for subtle differences in macro-molecular content statistically insignificant at a magnetic field strength of 1.5T.
Thus, these known techniques do not provide a tissue differentiation method that would enable evaluation of the combined effects of diffusion and magnetization transfer on normal brain parenchyma and pathological tissues and thereby obtain a quantifiable differentiation of tissues based on macromolecular content.